1. Field of the Invention
The invention relates to a resonator designed for narrow-linewidth emission, and particularly to a resonator for an excimer laser having optical components for improving spectral purity, reducing spectral bandwidth and optimizing output power for emitting a high resolution photolithographic beam.
2. Discussion of the Related Art
To increase the capacities and operation speeds of integrated circuits, manufacturers are inclined to design smaller internal structures for devices and other components of these chips. The reduction in size of a structure produced on a silicon wafer is limited by the ability to optically resolve the structure. This resolution ability depends directly upon the photolithographical source radiation and optics used.
Excimer lasers emitting pulsed UV-radiation are becoming increasingly important instruments in specialized material processing. The term "excimer" was first utilized as an abbreviation for "excited dimer", meaning two or more identical atoms comprising a molecule which only exists in an excited state, such as He.sub.2 and Xe.sub.2. Today, the term "excimer" has a broader meaning in the laser world and encompasses such rare gas halides as XeCl (308 nm), KrF (248 nm), ArF (193 nm), KrCl (222 nm), and XeF (351 nm). Several mercury-halides are also used as active gases in excimer lasers, such as HgBr. Even N.sub.2, N.sub.2.sup.+, CO.sub.2 and F.sub.2 (157 nm) may be used as active media within excimer laser discharge chambers. As is apparent, many excimer lasers radiate at ultraviolet wavelengths making them desirable for use as lithography tools. The KrF-excimer laser emitting around 248 nm and the ArF-excimer laser emitting around 193 nm are rapidly becoming the light sources of choice for photolithographic processing of integrated circuit devices (IC's). The F.sub.2 -laser is also being developed for such usage and emits light around 157 nm.
To produce smaller feature sizes on IC chips, stepper and scanner machines are using expensive large scale submicron projection objectives for imaging a reticle onto a wafer surface with high diffracting-limited precision. The objectives operate at deep ultraviolet (DUV) wavelengths, such as the emission wavelengths of excimer lasers. For example, the KrF-excimer laser emitting around 248 nm is currently being used as a DUV radiation source. To reach greater resolution limits, the large field objective lenses are designed and optimized in view of various possible and discovered imaging errors. The design optimization of the objectives is, however, inadequate to meet the precision demands of sub-quarter micron lithographic technology.
One way to improve the resolvability of structures on IC chips is to use more nearly monochromatic source radiation, i.e., radiation having a reduced bandwidth, .DELTA..lambda.. Other strategies include using shorter absolute wavelength, .lambda., radiation such as that emitted around 193 nm and 157 nm by ArF- and F.sub.2 -lasers, respectively, and increasing the numerical aperture (NA) of the projection lens.
The smallest structure resolvable on an IC chip depends on the "critical dimension" (CD) of the photolithography equipment being used: ##EQU1##
NA is a measure of the acceptance angle of the projection lens, .lambda. is the wavelength of the source radiation, and K.sub.1 is a constant around approximately 0.6-0.8. Simply increasing the numerical aperture NA to reduce the critical dimension CD simultaneously reduces the depth of focus DOF of the projection lens by the second power of NA: ##EQU2##
K.sub.2 is a constant around approximately 0.8-1.0. This complicates wafer adjustment and adds further strain on the demand for improved chromatic correction of the projection lenses. Additionally, increasing the numerical aperture NA to reduce the critical dimension CD for achieving smaller structures requires a decrease in the bandwidth .DELTA..lambda. of laser emission according to: ##EQU3##
K.sub.3 is a constant dependent on parameters associated with the projection lens(es). Each of the above assumes that such other laser parameters as repetition rate, stability, and output power remain constant.
Some techniques are known for selecting and for narrowing laser emission bandwidths including using optically dispersive elements such as etalons, gratings and prisms, as well as modified resonator arrangements. See U.S. Pat. No. 5,095,492 to Sandstrom (disclosing a dispersive grating having a concave radius of curvature); U.S. Pat. No. 5,559,816 to Basting et al. (disclosing a technique using the polarization properties of light); U.S. Pat. No. 5,150,370 to Furuya et al. (disclosing a fabry-perot etalon within the laser resonator); U.S. Pat. No. 5,404,366, U.S. Pat. No. 5,596,596 and E.U. Patent Pub. No. 0 472 727, each to Wakabayashi et al. (disclosing a concave outcoupler and a fixed aperture within the laser resonator); U.S. Pat. No. 4,829,536 to Kajiyama et al. (disclosing angularly offset etalons).
Using this available knowledge, the bandwidth of laser emission, e.g., which is naturally around 500 pm for a KrF-excimer laser, can be reduced to .DELTA..lambda..apprxeq.0.8 pm, sufficient to meet the demands of current projection lenses (NA.apprxeq.0.53) for producing quarter micron ship structures. Further improvements in projection objectives (NA.apprxeq.0.8) combined with a further reduction in laser emission bandwidths (.DELTA..lambda..apprxeq.0.4-0.6 pm) are expected to reduce the critical dimension CD using KrF-excimer laser sources down to CD.apprxeq.0.18 microns. See J. Mulkens et al., Step and Scan Technology for the 193 nm Era, Third International Symposium on 193 nm Lithography, Onuma, Japan (Jun. 29-Jul. 2, 1997).
The drawback to this significant bandwidth and CD reduction is a correspondingly significant reduction in available laser output power. Narrow band efficiencies of twenty to forty percent of broadband output power are typical. There is thus a need for efficient spectral narrowing methods which minimize power loss.
FIG. 1 shows a conventional excimer laser arrangement. A laser tube 1 contains a laser active medium (not shown) for emitting a characteristic wavelength upon excitation pumping of the laser active medium. A wavelength selection and narrowing assembly 2 includes a dispersive grating 3 and at least one expanding and/or dispersive prism 4. The grating 3 also serves to reflect substantially all of the laser light incident upon it at a wavelength dependent angle. A narrow band of the light dispersed once through the prism 4 and incident upon the grating 3 is reflected off of the grating 3 and back along the optical path of the arrangement, while all other wavelengths are reflected away from the optical path. The arrangement is completed with an output coupling mirror 5 which reflects a portion of the resonating band and allows the rest to continue unreflected ultimately defining the output beam of the system.
The excimer laser arrangement of FIG. 2 includes all of the elements of FIG. 1 except the output coupling mirror 5, and further includes a beam splitter 6 and a highly reflective mirror 8. The beam splitter 6 serves as an output coupler reflecting the narrow band laser emission 9 from the optical path of the resonating beam. A highly reflective mirror 8 is used instead of the partially reflecting output coupling mirror 5 of the arrangement of FIG. 1.